Sweden has called in Russia's ambassador to explain comments by a Foreign Ministry spokeswoman who said any Swedish decision to join NATO would have "consequences" that would compel Russia to respond.
"It is not acceptable to make threats," Swedish Foreign Minister Margot Wallstrom told reporters. "We are an independent country which makes independent decisions on our security policy."
Tensions have been mounting recently, with Sweden increasingly concerned by Moscow's actions in Ukraine and military activity around the Baltic region.
On Thursday a Russian Foreign Ministry spokeswoman said Sweden's admission to NATO "would have military, political and foreign policy consequences that would require indispensable response measures from the Russian side."
Sweden is not a member of NATO but is closely allied to the bloc and regularly takes part in military exercises.
The deteriorating security situation in eastern Europe has led to increasing calls for Sweden and neighbor Finland, which shares a 1,300-kilometer border with Russia, to formalize their relationships with NATO.
The two Nordic countries have increased military cooperation, with each other and NATO, but public support for membership remains relatively low.